When light hits chlorophyll a few things can happen


When pigments absorb photons an election gets excited


Light energy to chemical energy through photosystems (II & I)

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle (in stroma) uses products from light reaction


Calvin cycle is a 3-step process to produce sugars

- 1. Fixation: CO2 reacts with 5C molecule (RUBP)
- new 6C split into 2 acids (3C each)
- catalyzed by
Rubisco enzyme
- 2. Reduction: ATP and NADPH turn 3C acids into sugars
- reduction of acids
- from donation of electrons
- 3. Regeneration: Some 3C sugars recycled to make RUBP (5C)
- one 3C sugar leaves to make glucose
- recycling also needs ATP
Making sugars requires several cycles (for regeneration)


Rubisco - the enzyme that dominates carbon fixation

- Most abundant enzyme in the world
- Most CO2 converted into biomass is fixed by Rubisco
- Large molecule : 16 polypeptides : 8 active sites
Rubisco: the clunky and slow carbon fixer
- Only 3-10 reactions per second
- limits rate of photosynthesis
- 20% error rate
- gets worse at high temperatures
- Also reacts with O2 (cheater!)
- O2 reaction called
Photorespiration
- uses ATP and NADPH to make CO2

Photosynthesis is not perfect
- Evolution doesn’t always create the best solution
- Photorespiration is completely wasteful
- 2x energy to produce the same amount of sugar than if Rubisco only reacted with CO2
- Rubisco has evolved high affinity for CO2 (80x > O2)
- compare to the composition of the atmosphere…
- C3 photosynthesis is dominant, but other types of evolved

Rubisco and plant evolution: How did we get here?
- Why is there so much Rubsico?
- Why is it so big and slow?
- Was Rubisco always a cheater?
- Why did the atmosphere change?

Why has Rubisco not been replaced?
- Evolution of enzymes difficult
- favors stability
- selection against modifying mutations
- RubisCO → RubisC???
- requires a decrease in fitness
- plants are stuck with Rubisco
- Natural selection increased affinity for CO2

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